1. Field of the Invention
The present invention relates to an optical switch in a node device constituting a photonic network.
2. Description of the Related Art
A wavelength-division multiplexing (WDM) system is being developed and commercialized as a communications system for rapidly increasing the transmission capacity. To connect WDM systems and to organize a wide photonic network, a ring type network for connecting a plurality of devices in a loop shape is being studied. In this network, as the scale of a network is extended, a transmitting data amount in a loop is extremely increased. Thus, a mesh type network for connecting a plurality of devices in a mesh shape is also extensively studied.
The key technology of such a network organization is a large-scale optical switch for switching a lot of input/output fibers. In a ring type network or a point-to-point system, an electric switch is used to extract a low-order group signal from a node. By replacing this electric switch with an optical switch, cost of a node device can be reduced. Therefore, the implementation of a large-scale optical switch is a major issue in a variety of networks.
Most of currently commercialized optical switches are waveguide type small-scale switches and the switch is composed of an input/output fiber array and a switch element. To extend the scale of a switch element, the yield of a switch cell must be improved. However, since the dimensions tolerance in a manufacturing process is small, it is difficult to improve the yield.
Although an optical loss factor includes both loss in a switch cell and loss in a joint with an input/output fiber, it is difficult to reduce both the loss. For this reason, to extend the scale of a switch element, it is necessary not only to improve the yield by the improvement of the manufacturing process, but also to remarkably improve the element performance.
As a conventional art, there is also a configuration for spatially switching light. In this case, for example, if a reflection mirror is used as an optical path modifying element, there is no problem in performance, such as an on/off ratio, crosstalk and the like, which are problems in a waveguide type switch. However, since the volume of a switch increases, it is difficult to extend the scale from the viewpoint of size.
To break the deadlock, recently a semiconductor manufacturing technology for manufacturing this spatial switch in small size has been developed. This technology is called a xe2x80x9cmicro-electro-mechanical system (MEMS) and in particular the technology applied to the optical field is also called an xe2x80x9coptical MEMSxe2x80x9d. According to this technology, a micro-mirror is manufactured on a substrate by a semiconductor manufacturing technology and desired input and output can be spatially coupled by three-dimensionally inclining this mirror by static electricity.
FIG. 1A shows a MEMS micro-mirror. The size of a round mirror 11 is approximately several hundred microns and the inclination of this mirror 11 is adjusted by the static electricity of four electrodes 12 around the mirror 11. Thus, the output direction of light reflected on the mirror 11 three-dimensionally changes and switching is performed.
Since this spatial switch uses a mirror, the switch is superior to a waveguide type switch in switching performance, and the size is also as small as that of the waveguide type switch. Since in this way, one-input/n-output switching can be spatially performed by such one movable mirror, as shown in FIG. 1B, this switch is called a xe2x80x9cthree-dimensional MEMS switchxe2x80x9d.
In FIG. 1B, a movable mirror 13 can output light from one input fiber 14 to one of n output fibers 15. In this case, mirror displacement parameters are two of xcex8x representing the x-axis rotation angle of a mirror and xcex8y representing they-axis rotation angle. By applying prescribed voltages Vx and Vy to the electrodes, the inclination angle of a mirror is changed by a predetermined amount, and switching is performed accordingly.
If a large angle displacement amount of this movable mirror cannot be secured, as shown in FIG. 1C, movable mirrors can also be located in two stages. In FIG. 1C, light from the input fiber 14 is reflected on a first-stage movable mirror 16 and is directed to a fixed mirror 17. The light reflected on the fixed mirror is directed to a second-stage movable mirror 18. Then, the light is reflected on the movable mirror 18 and is outputted from one of n output fibers 15. In this case, since the mirror displacement parameters of each movable mirror are two of xcex8x and xcex8y, the total number of parameters becomes four.
If an n-input/n-output switch is configured, n and 2n movable mirrors are used in one-stage and two-stage types, respectively.
However, the conventional optical MEMS switch described above has the following problem.
The rigidity of a mirror used for switching varies depending on the ambient temperature and humidity. Due to this, the voltage-rotation angle characteristic of the mirror can change and an optical-coupling characteristic at the time of switching can degrade accordingly. The degradation of the optical-coupling characteristic includes the reduction of optical-coupling efficiency, crosstalk to another channel and the like.
The optical-coupling characteristic at the time of switching can also be degraded by the mechanical vibration of a mirror and the like. The degradation of an optical-coupling characteristic caused by such factors must be by any means compensated for.
It is an object of the present invention to provide a MEMS switch for compensating for the degradation of an optical-coupling characteristic in a node device constituting a photonic network.
The optical switch of the present invention comprises a mirror, the inclination angle of which varies depending on an application voltage, a driver device, an oscillation device, a superimposition device, a detection device and a control device. The driver device applies an application voltage to the mirror, and the oscillation device generates an additional signal of a prescribed frequency. The superimposition device superimposes the additional signal on the application voltage, and the detection device detects an signal component of the prescribed frequency from light reflected on the mirror. The control device performs control of the application voltage based on the detected signal component.